The present invention relates generally to the operation of pulverized fuel-fired combustion systems and, more particularly, to the pulverizing of high moisture solid fuels such as sub-bituminous coal, lignite and peat.
In suspension firing solid fuel, such as coal, lignite and peat, the fuel must be first pulverized and dried before it can be introduced into the furnace in a stream of air commonly termed primary air. This is accomplished in a mill wherein the solid fuel is simultaneously pulverized and dried in the primary air. In order to obtain sufficient heat for adequate drying of the fuel, the primary air must be preheated from ambient temperature to a higher temperature sufficient to provide adequate heat for drying of the pulverized fuel. At the same time, however, the heat content of the preheated primary air, i.e., the temperature of the primary air, must be kept below and not allowed to exceed that point at which excessive release of volatiles from the fuel would occur within the pulverizer as such an excessive release of volatiles could result in an explosion within the pulverizer.
Therefore, it is common in the prior art to bypass a portion of the primary air which is being supplied to the air preheater at ambient temperature around the air preheater and to remix this ambient tempering air with the preheated primary air from the air heater at a point upstream of the mill. The quantity and temperature requirements of the primary air stream to the mill are met by adjusting dampers disposed in the ambient tempering air and preheated primary air supply ducts prior to remixing of the ambient and preheated primary air streams.
Typically, the temperature of the mixture of pulverized fuel and primary air leaving the mill is measured and compared to a desired value. Simultaneously, the quantity of primary air entering the mill is sensed and compared to a preselected minimum level representative of the quantity of primary air necessary to insure an adequate transport velocity within the piping from the pulverizer outlet to the furnace so that the pulverized coal will not precipitate out of the air stream during transport. Each of the air dampers in the tempering primary air and the preheated primary air ducts are then operated in response to both airflow and temperature measurements to provide a sufficient quantity of primary air at a sufficient temperature.
Therefore, proper pulverizer operation requires that airflow be kept above a minimum commensurate with adequate transport velocity within the pulverized fuel piping while maintaining a temperature which is sufficiently high to dry the coal but not so high as to present a fire or explosion hazard within the mill. However, a problem has arisen when pulverizing very high-moisture fuels such as high-moisture bituminous coals, lignite and peat, in that adequate drying of the pulverized fuel cannot be obtained at the required primary airflow because the temperature of the primary air to the mill cannot be maintained at sufficiently high levels even with the ambient temperature tempering air duct dampers closed due to leakage of ambient temperature air around the dampers. Such leakage typically amounts to 10% of the total primary airflow to the mills. This problem is particularly troublesome in the winter in the temperate climates as the ambient temperature can typically be as low as -15 C. and even lower in some situations. When this very cold ambient air leaks through the tempering air duct and remixes with the preheated primary air, the temperature of the primary air entering the mill can be lowered by as much as 30 to 50 C. thereby resulting in inadequate drying of the high moisture fuel.
Therefore, it is the general object of the present invention to provide an improved method and apparatus for insuring that a sufficiently high primary air temperature can be maintained at the required primary air quantity to assure the adequate drying of very high moisture fuels.
It is a specific object of the present invention to provide a method and apparatus for heating any ambient temperature air leaking through the tempering air duct prior to remixing the tempering air with the preheated primary air thereby negating the effect of mixing cold ambient air with the preheated primary air leaving the air heater.